The present invention relates to microwave filters, and more particularly to frequency selective superconducting microwave filters. Many electronic systems, such as radar and communication systems operate over a wide range of microwave frequencies. In fact, many radar and communications systems operate in what is known as a frequency hopping or a frequency agile manner over very wide microwave (i.e., RF) bandwidths. The bandwidth can vary up to an octave or more and the frequencies can range from the S to Ku bands.
FIG. 1 illustrates the organization of a system's RF bandwidth 10. Radar agile frequency slots 15 are distributed throughout the system RF bandwidth 10. Interfering signals 20 are shown as black bars in FIG. 1, and reference numeral 25 identifies a particular radar signal. If the interfering signals 20 have sufficient amplitude and/or are sufficiently close to a particularly radar signal 25, interference may occur. The interference may result from, for example, intermodulation between two or more of the interfering signals 20, or by cross modulation between an interfering signal 20 and a particular radar signal 25.
Generally, radar and communications systems include nonlinear circuits that are positioned in a signal path prior to filtering that defines the instantaneous system bandwidth. These nonlinear circuits include, for example, low noise amplifiers and mixers. Increasing the linearity of such circuits tends to eliminate the effect of cross modulation between an interfering signal 20 and a particular radar signal 25. Avoiding the effects of cross modulation can also be achieved by preselecting the signal received by the system. Preselection can be achieved through the use of a switchable filter bank or a tunable filter.
Increasing the linearity of the nonlinear circuits, however, tends to increase their power consumption and cause a loss of sensitivity in these circuits. This is because the increased linearity is normally achieved by increasing the third order intercept point of these circuits. There is a direct relationship between the noise figure of a circuit and the third order intercept point of the circuit. Thus, increasing the linearity of the nonlinear circuits is not a desirable solution in most radar and communications systems.
Preselection or the use of filters ahead of the nonlinear circuits requires that the filters have very low loss, e.g., approximately 1 dB or less. Such filters would have a minimal impact on the sensitivity of the system. The filters must also have a very large dynamic range and a minimal bandwidth; that is, a bandwidth wide enough to pass a particular radar signal 25 and to reject interfering signals 20. Generally, this requires that the filters have a 1 to 2 percent, bandwidth. Currently, only waveguide filters are able to meet these requirements. Waveguide filters, however, are physically large and expensive.